The present invention relates to mitigating the toxic effect of metals in contaminated sites, and more particularly, to using stabilized nanoparticles for the in situ immobilization and/or containment of such toxic metals in contaminated sites.
Poor material handling as well as disposal practices of toxic metal containing materials has resulted in contamination of soils, sediments and groundwater. However, these metals are also typically added to animal feeds to meet their nutritional needs, and can thus enter the environment in animal waste. The most toxic metals are the so-called “heavy metals” such as arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc.
Mercury (Hg) is one of the most pervasive and bio-accumulative contaminants. The annual global input to the atmosphere is estimated to be 5,500-6,000 metric tons, of which 50-75% result from human activities. The annual anthropogenic Hg emitted in the U.S. totals 158 metric tons, of which about 33.3% are deposited in the homeland. In addition, the global reservoir adds about 35 tons of Hg annually to the U.S. territory. About 87% of the anthropogenic Hg emission is due to combustion, and about 10% to manufacturing industries.
Mercury contamination is a growing global problem. Since the industrial revolution, Hg content in the atmosphere has increased 200 to 500%. Hg concentration in Atlantic Ocean water has been growing at 1.2 to 1.5% per year since 1970, and Hg in Atlantic sea bird feathers was found to increase at 1.1 to 4.8% per year. Based on an analysis of 97 ice-core samples from the upper Fremont glacier, Wyo., Hg deposition has increased 20 fold over the last 100 years. EPA estimated that some 200,000 tons of Hg has been emitted since 1890, of which about 95% reside in soils or sediments, about 3% in ocean surface waters, and about 2% in the atmosphere.
Traditionally, the chlorine and caustic soda (i.e., Chlor-Alkali plants) industries and the earlier use of Hg in mining were the largest Hg sources. Because of its unique properties (e.g. high density, high surface tension, liquid at room temperature, toxicity, and volatility), Hg has been used in hundreds of industrial processes. However, these unique properties have also made Hg difficult to contain and recover. In fact, significant environmental releases have been detected in virtually all uses of Hg. For example, Chlor-Alkali plants had to replace 5-10% of their Hg stocks annually, with many facilities unable to account for 50% of the annual losses. In many cases, Hg has also been used indirectly by industries, to test materials (e.g., porosimetry), to measure processes (e.g., manometers), or as functional components (e.g., switches and seals). Starting in the early 1980's, industrial consumption and releases of Hg have been greatly curtailed as a result of increasingly stringent environmental regulations. However, the past releases of Hg have left a contamination legacy that has led to continuing releases of Hg to the atmosphere and surrounding groundwater and surface water bodies.
When it enters water and sediments, Hg can undergo a number of complex chemical and biological speciation and transformation processes, of which Hg methylation has been the primary environmental concern. Methylated mercury (or methylmercury, MeHg) can accumulate along the aquatic food chain, reaching its apex in predatory fish, where concentrations may be up to one million times higher than in the water column. As a result, even small concentrations of Hg in the water column (ng/L) can lead to significant concentrations of methylmercury in fish and waterfowl.
Mercury is a potent neurotoxin. Most at risk are children and the unborn. According to the CDC, one in 12 women of childbearing age has blood mercury levels exceeding the EPA safe level for protection of the fetus, i.e. about 320,000 babies born annually in the U.S. are at risk for neuro-developmental delays. In wildlife, mercury is a productive hazard with harmful effects on a variety of species.
Triggered by the toxicity and bioaccumulation concerns, the U.S. EPA has identified Hg as one of its twelve priority persistent bio-accumulative toxins (PBTs). Meanwhile, a number of recent programs and plans have set water quality objectives for Hg in surface waters to <50 ng/L, far below the concentrations in surface waters draining most industrial facilities. As of 2003, EPA, FDA and 45 states have issued about 3089 fish consumption advisories, of which 80% are, at least in part, associated with Hg poison. Because of the heavy Hg hit, 100% of the Gulf coast line is covered by the advisories. These advisories have caused tremendous economic less in aquacultures and are continuingly hurting the growing aquacultures.
While knowledge on the biogeochemistry of Hg is still growing, there is a general consensus that there are two keys toward control of Hg poisoning: 1) reduce the source of Hg emission, and 2) minimize Hg methylation. Over the last two decades, Hg usage has been cut down dramatically in the U.S. as a result of tightened environmental regulations. However, there has been little progress in developing engineered remediation processes to control Hg methylation.
Traditionally, remediation of Hg-contaminated soils or sediments employs excavation and subsequent disposal in a landfill. However, this method is very costly and environmentally disruptive, and the landfilled Hg will very likely leach back to the environment. Recently, phytoremediation was explored to remove Hg from soils. For instance, some water hyacinths from South America were able to concentrate 4,435 ppb Hg in their roots and 852 ppb Hg in their shoots. However, this promising technology is held back by the common questions of what to do with the Hg-saturated plants, which are a hazardous waste and are subject to the RCRA. Biological conversion of methylmercury to elemental Hg was used to treat wastewater streams from Chlor Alkali plants. However, the de-methylation process is not favored in the subsurface environment.
Permeable reactive barriers (PRBs) have been employed to remediate various contaminated sites. Typically, scrap iron (i.e., Fe) is employed to in situ immobilize various redox active metals in soils and groundwater. However, because of the large particle size, the reaction kinetics if very limited. As a result, when applied to Hg immobilization, conventional PRBs have been seen to stimulate the growth of sulfate-reducing bacteria (SRB), the primary culprits of Hg methylation.
Lead is also a widespread contaminant in soils and groundwater. Lead has been ranked the second most hazardous substance in the U.S. by the Agency for Toxic Substances and Disease Registry (ATSDR) and the U.S. Environmental Protection Agency (USEPA) (ATSDR, 2005). In 1999, lead was identified as a major hazardous chemical at 47% of the 1,219 Superfund sites on the USEPA's National Priorities List (USEPA, 1999). Current remediation technologies for contaminated soil remediation are rather costly and/or often environmentally disruptive. Consequently, innovative remediation technologies for controlling lead-poisoning are urgently needed.
In recent years, in situ immobilization of Pb2+ in contaminated soils with phosphate-based amendments has elicited a great deal of attention. This approach reduces the Pb2+ mobility, and thus toxicity, by transforming the labile form of Pb2+ in soils to the geochemically more stable pyromorphites (Pb5(PO4)3X, where X═F, Cl, Br, OH) by amending contaminated soils with soluble phosphate salts or solid phosphate minerals such as apatite. Pyromorphites are considered as the most stable forms of Pb2+ under a wide range of environmental conditions, and are over 44 orders of magnitude less soluble than other common Pb2+ minerals in contaminated soils such as galena (PbS), anglesite (PbSO4), cerussite (PbCO3), and litharge (PbO). For in situ immobilization of Pb2+ in soils, phosphate has been commonly applied to soils in its soluble forms such as phosphoric acid, NaH2PO4 or KH2PO4, or in solid forms such as synthetic apatite, natural phosphate rocks, and even fishbone (with apatite being the effective composition). Among those additives, phosphoric acid was regarded as the most effective amendment (USEPA, 2001) for its easy delivery and superior ability to dissolve Pb2+ from existing minerals and transform it to pyromorphites. Amendment dosage of 3% PO43− by weight for soils has been proposed and applied by USEPA and other government agencies (USEPA, 2001).
However, adding large amounts (e.g. the 3% PO43− dosage) of highly soluble phosphoric acid or phosphate salts into the subsurface is limited by not only the material cost but the secondary contamination problems. Due to the high solubility of phosphate, groundwater and surface waters in the affected area may be contaminated by excessive nutrient input. To avoid phosphate leaching, solid phosphate (e.g. rock phosphate) was also studied. However, effectiveness of solid phosphate is hindered by the large size of the particles. In fact, even fine-ground solid phosphate particles are not mobile in soils, which prevents solid phosphate from being delivered to the lead-affected zone and from reaching and reacting with Pb2+ sorbed in soils.
In recent years, environmental application of nanoscale zero-valent iron (ZVI) has attracted considerable interests. In addition to reductive dechlorination uses, ZVI nanoparticles have been studied for transformation of inorganic contaminants such as chromate (CrO42−), arsenate (AsO43−), perchlorate (ClO4−), and nitrate (NO3−). Compared to conventional powder or granular iron particles, nanoscale ZVI particles offer improved reactivity. However, ZVI nanoparticles can form micron-scale agglomerates rapidly, resulting in loss in soil mobility and reduced reactivity. To prevent nanoparticle agglomeration, a strategy to stabilize ZVI-nanoparticles using a low-cost and environmentally friendly cellulose (CMC) has been developed as a stabilizer. The stabilized ZVI nanoparticles displayed much improved reactivity as well as soil mobility compared to non-stabilized counterparts.